The present invention relates to a process and a device for purifying in particular substantially organically polluted waste water.
In connection with a process and a device for distilling raw water, such as is known from DE 26 00 398 C2, a dual cycle system is used, to which a heat pump cycle process is coupled for use as an evaporation process. On the one hand, this has the advantage that it becomes possible to generate a qualitatively high-grade, i.e. highly pure, distillate, such as is demanded in the pharmaceutical, biotechnical and electro-technical industries and that on the other hand this can be attained in a very efficient manner. In the course of the distillation of raw water the concentrated raw water sump solution is drawn off approximately once a month, which is not mentioned in the noted reference. This is sufficient because of the negligible pollution of the raw water, if it is of drinking water quality, because a dual cycle process is practically not disadvantageously affected during this time. However, this results in a discontinuous process, which is disadvantageous.
In accordance with an article by J. Hoi.beta. in "Zeitschrift fur wirtschaftliche Fertigung" Journal of Commercial Manufacturing!, Vol. 79, 1984, pages 141 to 145, tests were performed as to how the process or the device would react, if they or it were used for purifying nitrate-nitrite-containing wash water from hardening factories. The aim of the tests was in particular to determine whether there would be a dependency of the quality of the distillate on the strength of the concentrated nitrate-nitrite-containing wash water sump solution. Because a decrease in the quality of the distillate as a function of this sump solution concentration was found, two possibilities for operating the process were cited in this article, namely continuous operation with approximately ten to fifteen percent sump draw-off or a discontinuous operation up to the point where the sump had been concentrated to the point where the power consumption of the device had reached limit values inherent in the process. The second one of the mentioned possibilities suffers from the above mentioned disadvantage of discontinuity, which implies stopping the device, removal of the waste sump solution, cleaning of the device and possibly removal of deposits on the device. It is also necessary to monitor the quality of the distillate continuously throughout the process so as to be able to stop the process even at an inconvenient time. This is relatively expensive. Although the above mentioned first possibility results in the continuity of the process operation, it requires that such nitrate-nitrite-containing wash waters are always of the same "quality" in order to be able to fix the percentage value of the continuous sump draw-off.
Now, if it is intended to purify or distill industrial waste water, particularly substantially organically polluted waste water, the above mentioned possibilities of working the process cannot be applied easily, not least because the total pollution with chemically oxidizable matter, essentially organic pollution, for the measurement of which the so-called CSB value (value of chemical oxygen demand COD!) is of importance, can vary considerably. Primarily it must be assumed that a discontinuous operation or processing is out of the question because of the above mentioned disadvantages. The continuous process operation with continuous draw-off of the concentrated waste water sump solution is not practically feasible for the following reasons: Such waste water constitutes a mixture of many materials containing many problem materials with different behavior in addition to water. Such problem materials are, for example, low boiling components, the boiling temperature of which therefore lies considerably below that of water, and foam-generating components, which can make distillation of the waste water questionable. The low boiling components, for example, generate gas and form gas pockets in places where the condensation of the waste water vapor is to take place. The results of this are irregularities in the connected heat pump cycle process, so that the heat balance of the process or in the device is no longer correct because of possible different specific heat transfers in the individual process steps. This can lead to an increase in overheating in the secondary system and thus to too high a temperature, which must result in shutting down the process or the device in order to prevent damage. The foam-generating components of such waste water act in a similar manner, namely because of the fact that they can clog the process step where the waste water evaporates and can paralyze the process step in which the waste water vapor is purified in a cyclone.